By restricting the domain of \(\partial c_{\text{cont}}/\partial x\)  to the original unit interval, the numerical derivatives of \(c\)  are hence approximated to high-order. The overall errors are found only in the production of Equation (REF ), from where the calculation of the derivative Equation(REF ) is easily facilitated by a Fast Fourier Transform (FFT). This overall idea can also be easily extended to any general interval \([x_0,x_N]\)  via affine transformations [1]}.
